The invention concerns an NMR (nuclear magnetic resonance) probe head comprising a microwave resonator having at least two elements which are reflective in the microwave range, at least one of which is focusing, wherein the reflective elements at least partly delimit a resonance volume of the microwave resonator.
A configuration of this type is disclosed e.g. in US 2011/0050225 A1 (reference [2]) which describes the use of a Fabry-Pérot resonator (FP) in the field of DNP-NMR/EPR.
A Fabry-Pérot resonator typically consists of two parallel metallic mirrors located opposite each other at a separation L. In quasi-optical systems, one or both mirrors may have a spherical shape (with a radius of curvature Ri). The electromagnetic field distribution within such a resonator can be described by the paraxial solution of the Helmholtz equation which is based on Gaussian optics. The resonance frequency of the FP resonator depends on the radii of curvature R1,2, the mirror separation L and the desired resonance modes. In the special case of transverse electromagnetic modes TEMmnq a particular mode is unambiguously determined by the indices m, n, and q.
One possible implementation of an FP resonator consists of a spherical and a planar mirror in accordance with present FIG. 3, wherein in accordance with reference [1] the resonance frequency is given by
            f      r        ⁡          [              q        +                                            m              +              n              +              1                        π                    ⁢                                    (                                                tan                                      -                    1                                                  ⁢                                  L                                                            R                      1                                        -                    L                                                              )                                      1              2                                          ]        ⁢      c          2      ⁢                          ⁢      L      
As is illustrated in the left half of FIG. 2, a mirror element formed as a stack of dielectric layers with different permittivities is also disclosed in optics as an alternative to a metallic mirror. Such a structure is designated as distributed Bragg reflector (DBR) in optical configurations. The reflectivity of a DBR is determined by the number and permittivity of the dielectric layers as
  R  =            [                                                                  n                0                            ⁡                              (                                  n                  2                                )                                                    2              ⁢              N                                -                                                    n                s                            ⁡                              (                                  n                  1                                )                                                    2              ⁢              N                                                                                          n                0                            ⁡                              (                                  n                  2                                )                                                    2              ⁢              N                                +                                                    n                s                            ⁡                              (                                  n                  1                                )                                                    2              ⁢              N                                          ]        2  n0, n1, n2, and ns correspond to the refractive indices of the background material, the alternating layers and the substrate material (cf. e.g. reference [4]).
There are experimental methods in the field of nuclear magnetic resonance spectroscopy which enable a significant increase in the nuclear polarization and therefore in the detection sensitivity of the experiment. One of these methods is dynamic nuclear polarization (DNP). This technology is based on the excitation of electron spins in stable radicals and, due to the gyromagnetic relationship of the electron spin, requires simultaneous irradiation of a magnetic microwave field at a frequency which is higher by a factor of 660 than the nuclear Larmor frequency of the 1H nuclei.
A typical DNP configuration consists of an NMR coil which is tuned to a nuclear Larmor frequency (e.g. 1H—400 MHz) and simultaneous irradiation of a microwave field at 263 GHz. DNP configurations are described e.g. in reference [3].
The major problem of the current prior art consists in that the sample should be located as closely as possible to the NMR detection coil while being excited as homogeneously as possible by means of a microwave field. While in the case of an FP resonator homogeneous microwave excitation is easy to realize, the presence of a conductive mirror in the direct vicinity of the sample causes two problems:
1.) The currents induced in the mirror distort the field of the detection coil and reduce the detection sensitivity.
2.) the transition between the metallic mirror and the sample interferes with the external (static) magnetic field, thereby reducing the spectral resolution of the experiment.
Departing therefrom, it is the underlying purpose of the present invention to provide a microwave resonator of the above-mentioned type which enables location of an NMR detection coil as closely as possible to the sample, wherein the distortions of the static field by resonator components are reduced such that the detection sensitivity and the spectral resolution of the experiment are considerably improved.